2004 Ford F150 momentary exhaust leak

[Capt. Delta]

This one's got me baffled. My Dad called me about an exhaust leak on his 04 F150. 5.4L v8.
When he first starts the truck there is an obvious exhaust leak noise coming from the underside but it only lasts 20-30 seconds and then stops abruptly. I'd have to think some kind of baffle is being activated for it to stop so suddenly. Any ideas from the wise brain trust out there in cyberland. Thanks in advance

[RahX]

You might have a crack or something along those lines that seals up when the exhaust heats up.

[Capt. Delta]

I thought of that but it goes from making the noise to stopping all at once. Just like someone closed a valve. I think a crack sealing would change tones as it got smaller. I've also never heard of an exhaust leak sealing itself but I appreciate the input

[RahX]

Hmm interesting. I'll have to check and get back with ya.